1. Field of the Invention
The present invention relates to cryogenic deflashing apparatus for removing flash and burrs from molded parts in a low temperature environment by the bombardment of the articles with a high velocity pellet or media stream and more particularly to a blast media transport and throwing wheel for use in such apparatus.
2. Description of the Prior Art
The molding of articles from plastic, rubber and other materials generally leaves a residual flash on the articles in the area adjacent the interfacing mold surfaces. Such flash is functionally and aesthetically objectionable. The removal of such flash by hand trimming is costly and sometimes difficult.
To eliminate the hand trimming operation deflashing apparatus has been developed which cools the flash through the use of liquid cryogens, e.g. N.sub.2, to the embrittlement point and then subjects the articles to a high velocity stream of pellets, commonly referred to as the blast or deflashing media.
Throwing wheels used for propelling the blast media against the articles to be deflashed are typically centrifugal devices similar in configuration to straight vane centrifugal fans. The blast media to be accelerated is introduced at the eye (i.e. an axial opening) of the centrifugal impeller or wheel by a feed cage. The media is accelerated by the wheel's radially extending vanes and directed in a preselected pattern against the articles. The width of the pattern is controlled by the width of the vanes while the length and direction of the pattern are controlled by the opening in the feed cage and its location. The shroud which surrounds the wheel serves to contain and direct the stray pellets. Unlike the flow of air into the impeller of a centrifugal fan, the feed cage restricts the flow of the media or pellets to only a fraction of the total circumference of the axial opening. See for example, U.S. Pat. No. 3,368,308 and application Ser. No. 918,707 filed June 26, 1978 (abandoned). Although such prior art throwing wheels have proven useful in their general application there are substantial deficiencies associated with their use.
In particular the prior art throwing wheels while meeting the requirement of accelerating the media to the required exit velocity also pump large amounts of air with the media. The pumped air serves no purpose in the deflashing process but does raise the power required to drive the throwing wheel substantially. The increased power not only results in a larger motor and armature current for the throwing wheel than would otherwise be necessary but also results in heating the atmosphere surrounding the article being deflashed. This added heat load must be removed by additional cooling which adds to the overall cost of the deflashing process.
The prior art throwing wheels are typically positioned above the articles to be deflashed with the feed cage and a feed tube (for channeling the media to the feed cage) extending horizontally from the throwing wheel. The feed cage is arranged to feed the media into an area above the rotational axis of the wheel. A conveyor mechanism typically in the form of a screw conveyor has been used to move the media horizontally along the feed tube, force it into the feed cage and up into the throwing wheel. However, the use of such a screw conveyor requires an additional prime mover to rotate the conveyor with its attendant cost, power requirements and maintenance.
There exists a substantial need in the art for a blast media transport and throwing wheel which eliminate the above-mentioned deficiencies.